This invention relates generally to a gear mechanism and more particularly to a gear mechanism having means for precise predetermined movement of a driven member and, further, having means capable of disengagement of gears to prevent damage to the gears during an occurrence of an abnormal load applied to the gear mechanism. The gear mechanism of this invention has application to a record medium feeding apparatus for feeding a record medium, such as paper, transparent film or thermal sensitive material in conjunction with a printer or the like.
Japanese Publication No. 50876/83 discloses the typical type of gear mechanism used in conjunction with a record medium feeding apparatus, such as employed in conjunction with a printer or the like. The gear mechanism is designed so that drive gear 9 intermittently drives the driven gear 8 for intermittent feeding of the medium to the printer. In the apparatus shown in this publication, the angular position of the shaft of driven gear 8 is determined by the position where roller shaft 4 comes into engagement with oval shaped hole 6-1 of bearing 6. However, the distance between the centers of drive gear 9 and driven gear 8 is determined by supporting member 1, bearing 6, roller shaft 4, and drive gear shaft 10. Since there are many interrelated components involved, there is the frequent occurrence of an aberration in and stability of gear engagement between driving gear 9 and driven gear 8 resulting in the frequent occurrence of skipped gear engagement. Also, when this type of gear mechanism is employed in such a medium feeding apparatus, it frequently operates in an intermittent, nonuniform fashion. Unless the distance between the centers of drive gear 9 and driven gear 8 is continuously maintained, the angular position wherein driven gear 8 is locked for driving engagement by drive gear 9 will vary from cycle to cycle. As a result, the incremental amount of fed medium from cycle to cycle is not the same and the time period of incrementally fed medium from cycle to cycle is not uniform. These problems become more exaggerated when hole 6-1 of bearing 6 becomes worn. As a result, adjacently printed characters on the medium will not be printed evenly and the desired standard of print quality is substantially reduced.
It is an object of the present invention to provide a gear mechanism wherein precise intermittent movement may be provided to a driven gear by a drive gear without the occurrence of skipped gear engagement.
Another object of this invention is a gear mechanism for a medium feeding apparatus providing for precision in the metered amount of intermittently fed medium in a printer as well as providing uniformity in the time period or cycle of intermittent feeding of precise amounts of medium.
It is another object of this invention to prevent damage to gears of a gear mechanism transferring power to a load during the occurrence of an abnormal load applied to the gear mechanism.